In the production of chemical pulp of high brightness, wood chips are first cooked to separate the cellulose fibres. During the cooking, part of the lignin holding the fibres together is degraded and modified such that it can be removed by subsequent washing. However, in order to achieve sufficient brightness, more lignin must be removed, together with brightness impairing (chromophoric) groups. This is frequently effected by delignification with oxygen, followed by bleaching in several stages.
Bleaching of chemical pulps is mainly carried out with chlorine-containing bleaching agents, such as chlorine, chlorine dioxide and hypochlorite, and furthermore in intermediate extraction stages with alkali and, optionally, hydrogen peroxide and/or oxygen. The chlorine-containing substances react with the lignin and render it soluble, either in the same stage or in the subsequent alkaline extraction stage.
A conventional bleaching sequence for softwood treated in accordance with the sulphate process is EQU (C+D) E.sub.1 D E.sub.2 D
wherein
(C+D)=addition of chlorine (C) and chlorine dioxide (D) in the same stage, simultaneously or sequentially PA1 D=chlorine dioxide stage PA1 E.sub.1 =alkaline extraction stage, optionally with addition of peroxide (EP) and/or oxygen (EPO and EO, respectively) PA1 E.sub.2 =alkaline extraction stage, optionally with addition of peroxide (EP)
The (C+D) and E.sub.1 stage is defined as a prebleaching stage. The sequence D E.sub.2 D is called final bleaching. The above-mentioned oxygen delignification is normally regarded as part of the cooking process.
The reaction products formed in the bleaching stages using halogen-containing chemicals, give rise to discharges containing, inter alia halogenated organic compounds. These compounds can be measured, inter alia as AOX (=adsorbable organic halogen). When using chlorine dioxide, the AOX formation is but a fifth of that obtained with molecular chlorine. With increasingly stringent environmental standards, the proportion of chlorine dioxide has therefore been increased in the last few years. Thus, it is becoming more and more common to increase the proportion of chlorine dioxide in the (C+D) stage, and it will not be long before a pure D stage, frequently termed D.sub.0, is used instead. To be able to compare the bleaching effect of the chlorine-containing substances, the amount of active chlorine is indicated, which is the oxidation potential multiplied by the amount of chlorine in the molecule, 1 kg chlorine dioxide corresponding to 2.63 kg active chlorine.
The cooking can be modified in several ways to further reduce the kappa number, before commencing the bleaching with chlorine-containing chemicals. In spite of these measures, the even more stringent restrictions which future environmental standards may be expected to impose on discharges will make it necessary to continue the processing of the effluents arisen. In the future, however, the water discharged into the receiving body of water must have an AOX content far lower than today. The method according to the present invention makes this practically feasible.
Today's biological techniques have primarily been developed to reduce the discharges of BOD (=biological oxygen demand), whereas e.g. the high-molecular fraction of AOX is not affected to any appreciable extent. The most common external biological purification techniques are the aerated lagoon and the activated sludge tank. In the aerated lagoon, the residence time is about 1-7 days, and the AOX reduction is limited to about 25-30%. This applies also to the COD content (=chemical oxygen demand). The waste water is conducted through an activated sludge tank in 12-24 hours, the AOX content being reduced by 40% or more, and the COD content by 40-60%. Present research is aimed also at developing biological techniques capable of degrading AOX molecules of high molecular weight. However, the future of these techniques is uncertain, inter alia because the types of fungi employed are difficult to separate intact.
Another technique that has been investigated is ultrafiltration which is most efficient in the processing of spent bleach liquor containing larger molecules. Such spent bleach liquor is obtained, inter alia from the first extraction stage.
According to a paper published by Bottger et al in "Das Papier", Vol. 40, 1986, No. 10A, pp V25-33, chlorine-bleached spruce and birch sulphite pulp is treated by an alkaline-thermal method for degrading chlorinated lignin compounds. The method was used on spent bleach liquor from the chlorine stage, and the AOX value before and after the treatment was determined. The results show that treatment at a pH of between 10.5 and 11.2, a temperature of between 60.degree. and 70.degree. C., and a residence time of 2 h, makes it possible to reduce the AOX content in the spent bleach liquor by a maximum of 54%. Such a high pH is undesirable for process reasons, and frequently impossible to obtain. Besides, the addition of sodium hydroxide means that the method is uneconomic, a fact also pointed out by the authors. Furthermore, the method has been applied to AOX formed in the bleaching of sulphite pulp. Normally, sulphite pulp is more easily bleached than sulphate pulp, which indicates that differences may exist between AOX formed when bleaching sulphite pulp and sulphate pulp, respectively.
DE 3,620,980 discloses a method of treating chlorinated organic compounds in the spent bleach liquor from an initial chlorine or chlorine-containing stage in a bleach plant for sulphite pulp. The process comprises three stages, the first stage involving increasing pH from 1.4 to about 6.2 and the simultaneous addition of calcium ions to facilitate precipitation. After the pH has been further increased to 10-12, the AOX content is reduced by precipitation of calcium salts. The second stage involves treating the filtered-off alkaline water at 40.degree.-70.degree. C. for 1-3 h. The final stage involves a microbial, biochemical treatment. It appears from the patent that a pH of about 11.5 is necessary in the second stage, which makes this technique expensive.